1. Field of the Invention
The present invention relates to focus control on an image-taking apparatus.
2. Description of the Related Art
A hybrid AF control method is available as an auto focus (AF) control method used for an image-taking apparatus such as a video camera. This hybrid AF control method combines a so-called TV-AF method which controls a focus lens so that an AF evaluated value signal obtained by extracting a high-frequency component (contrast component) from an output signal of an image-pickup device becomes a maximum and a control method which calculates a driving amount of a focus lens using a signal from a sensor which measures a distance to an object or an amount of defocusing (e.g., see Japanese Patent Application Laid-Open No. 2002-258147).
This hybrid AF control method achieves high focusing accuracy in the TV-AF method, however, it requires a long focusing time to search for the lens position at which the AF evaluated value signal becomes a maximum. Therefore, it is designed to shorten the focusing time with reference to a signal from a separately provided sensor.
Furthermore, Japanese Patent Application Laid-Open No. 2002-258147 proposes a hybrid AF structure which combines a TV-AF method and an internal measuring method in which incident light through an image-taking optical system is separated and received by a sensor. Then an amount of defocusing is measured based on the light signal received. Furthermore, the same Application also proposes a combination of a TV-AF method with an external measuring method which provides a distance sensor else an image-taking optical system.
The internal measuring method has an advantage of reliably capturing an object because imaging object-image is also formed on the sensor. However, since a spectroscopic mechanism needs to be provided in an image-taking optical path, the sizes of the lens barrel and the whole image-taking apparatus tend to increase. In recent years, there is a growing demand for miniaturization of a consumer image-taking apparatus such as a video camera, and in this sense, the internal measuring method which imposes limitation on the sizes and layout of components mounted is not desirable.
On the other hand, since the external measuring method only arranges a distance sensor independently of an image-taking optical system, it has a high degree of freedom in layout, and is therefore advantageous to reduce the size of the image-taking apparatus.
However, since the aforementioned external measuring method arranges the distance sensor independently of the image-taking optical system, there is a gap between the optical axis of the image-taking optical system and measuring axis of the distance sensor, producing a problem of so-called parallax. FIG. 2 shows a relationship between the image-taking range and the measuring range of the distance sensor when the image-taking apparatus zooms in toward the telephoto side.
Since the image-taking angle of view on the telephoto side is narrow, the distance sensor captures an object B which is different from an object A whose image is actually taken as shown in FIG. 2. For this reason, there is a possibility that the hybrid AF control will not function appropriately, such as not to achieve to focusing on the object or to need long time in focusing.
Especially, a digital camera or video camera in recent years is provided with an electronic zoom function which performs display and record enlarging an object image taken by an image-pickup device such as a CCD or CMOS sensor through electric processing. This electronic zoom function performs pseudo-zooming beyond the zoom magnifying ratio (on the order of 10× to 20× in a video camera) through electric processing, realizing zooming of up to several tens to several hundreds of times.
At this time, since the apparent image-taking angle of view narrows considerably, it becomes extremely difficult to reliably capture the object within the image-taking angle of view using a distance sensor, further increasing the possibility that the hybrid AF control will not function appropriately.